AM processes generally involve the buildup of one or more materials to make a net or near net shape (NNS) object, in contrast to subtractive manufacturing methods. Though “additive manufacturing” is an industry standard term (ASTM F2792), AM encompasses various manufacturing and prototyping techniques known under a variety of names, including freeform fabrication, 3D printing, rapid prototyping/tooling, etc. AM techniques are capable of fabricating complex components from a wide variety of materials. Generally, a freestanding object can be fabricated from a computer aided design (CAD) model. A particular type of AM process uses an energy directing device comprising an energy source that emits an energy beam, for example, an electron beam or electromagnetic radiation such as a laser beam, to sinter or melt a powder material, creating a solid three-dimensional object in which particles of the powder material are bonded together. Different material systems, for example, engineering plastics, thermoplastic elastomers, metals, and ceramics are in use. Laser sintering or melting is a notable AM process for rapid fabrication of functional prototypes and tools. Applications include direct manufacturing of complex workpieces, patterns for investment casting, metal molds for injection molding and die casting, and molds and cores for sand casting. Fabrication of prototype objects to enhance communication and testing of concepts during the design cycle are other common usages of AM processes.
Selective laser sintering, direct laser sintering, selective laser melting, and direct laser melting are common industry terms used to refer to producing three-dimensional (3D) objects by using a laser beam to sinter or melt a fine powder. For example, U.S. Pat. No. 4,863,538 and U.S. Pat. No. 5,460,758 describe conventional laser sintering techniques. More accurately, sintering entails fusing (agglomerating) particles of a powder at a temperature below the melting point of the powder material, whereas melting entails fully melting particles of a powder to form a solid homogeneous mass. The physical processes associated with laser sintering or laser melting include heat transfer to a powder material and then either sintering or melting the powder material. Although the laser sintering and melting processes can be applied to a broad range of powder materials, the scientific and technical aspects of the production route, for example, sintering or melting rate and the effects of processing parameters on the microstructural evolution during the layer manufacturing process have not been well understood. This method of fabrication is accompanied by multiple modes of heat, mass and momentum transfer, and chemical reactions that make the process very complex.
FIG. 1 is schematic diagram showing a cross-sectional view of an exemplary conventional system 100 for direct metal laser sintering (“DMLS”) or direct metal laser melting (DMLM). The apparatus 100 builds objects, for example, the part 122, in a layer-by-layer manner by sintering or melting a powder material 112 using an energy beam 136 generated by a source such as a laser 120. The powder to be melted by the energy beam is supplied by reservoir 126 and spread evenly over a powder bed 114 using a recoater arm 116 travelling in direction 134 to maintain the powder at a level 118 and remove excess powder material extending above the powder level 118 to waste container 128. The energy beam 136 sinters or melts a cross sectional layer of the object being built under control of the galvo scanner 132. The powder bed 114 is lowered and another layer of powder is spread over the powder bed and object being built, followed by successive melting/sintering of the powder by the laser 120. The process is repeated until the part 122 is completely built up from the melted/sintered powder material. The laser 120 may be controlled by a computer system including a processor and a memory. The computer system may determine a scan pattern for each layer and control laser 120 to irradiate the powder material according to the scan pattern. After fabrication of the part 122 is complete, various post-processing procedures may be applied to the part 122. Post processing procedures include removal of excess powder by, for example, blowing or vacuuming. Other post processing procedures include a stress release process. Additionally, thermal and chemical post processing procedures can be used to finish the part 122.
FIG. 2 shows powder bed for additive manufacturing according to the prior art. A powder dispenser 201 is provided that pushes an amount of powder (e.g., CoCr) upward into the build chamber where a roller or arm 202 spreads the powder over the build plate 203. A laser heats the powder in a desired pattern corresponding to a cross section of a part, sintering or melting the powder to form a solid cross section slice on the build plate 203. The build plate is lowered and the powder dispenser and roller or arm redistributes a thin layer of powder over the build plate. The laser then heats the powder building on the earlier deposited pattern of fused material, thereby making successive layers in the additive manufacturing process.
Powder beds are commonly used in laser bed additive manufacturing techniques. These techniques generally require a step of providing a thin layer of powder over a build plate within the additive manufacturing apparatus. In one example, a powder dispenser 201 is provided that pushes an amount of powder (e.g., CoCr) upward into the build chamber where a roller or arm 202 spreads the powder over the build plate 103. FIG. 1. A laser heats the powder in a desired pattern corresponding to a cross section of a part, sintering or melting the powder to form a solid cross section slice on the build plate 203. The build plate is lowered and the powder dispenser and roller or arm redistributes a thin layer of powder over the build plate. The laser then heats the powder building on the earlier deposited pattern of fused material, thereby making successive layers in the additive manufacturing process.
FIG. 3 shows a powder bed and recoating system for additive manufacturing according to the prior art. A powder hopper 301 is used in conjunction with a recoater arm/temporary hopper 302. The recoater arm/temporary hopper 302 spreads a thin layer of powder over the build plate 303 by moving across the build plate and dropping powder in a controlled manner to provide a thin layer of powder. This process is repeated with each laser writing step and lowering of the build plate in the additive manufacturing process.
A problem that arises in prior art systems and methods is that the recoater blade may become damaged during the build process. This is particularly problematic for large or complicated builds that may take a longer time and/or involve more detailed structures and features. In general, the longer a build takes, the more likely it is that the recoater blade becomes damaged. If the recoater blade is damaged, it could compromise the quality and integrity of the object being built. Using a damaged blade during at least part of a long, complicated build could require the final object be discarded and rebuilt from scratch. Even if the damage is discovered quickly, to replace the recoater blade in the prior art systems and methods generally requires the whole system be shut down and opened up. This results in work stoppage, delay, and expense. Therefore there is a need to exchange a damaged recoater blade for an undamaged one, without substantially delaying or stopping the additive manufacturing process.
Also, for some builds, it may be desirable to use a different recoater blade material while building one or more object(s). In general, it is preferable for the recoater blade to be the same material as the material being used to make the object. If more than one material is used during a build, either to make portion(s) of an object, or to make a separate object, this may require the system to be shut down and opened up, so that an appropriate recoater blade can be installed. This results in work stoppage, delay, expense, and additional oxygen exposure. Also, it may be preferable to use a different recoater blade for some builds and/or portions of some builds that require very fine features. Therefore there is a need to exchange one recoater blade for another during an additive manufacturing process, without substantially delaying or stopping the additive manufacturing process.